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For several years NASA's scientists failed to accept data on the Antarctic ozone hole that was before their eyes. The reason: computers prescreening data from monitoring satellites had been programmed to dismiss as suspicious presumably wild data showing a 30% or greater drop in ozone levels. After British scientists reported the deficit in 1985, NASA went back to its computer records, finally recognizing that the satellite data had been showing the hole all along.
Still, the existence of an ozone hole did not necessarily mean CFCs were to blame, and a number of alternative explanations were proposed. Among them, says Dan Albritton, director of the Federal Government's Aeronomy Laboratory in Boulder, was the notion that the "hole did not signify an ozone loss at all, just a breakdown in the distribution system." An interruption in the movement of air from the tropics, where most ozone is created, to the poles could easily result in less ozone reaching the Antarctic. Another theory: perhaps the sunspot activity that peaked around 1980 created more ozone- destroying nitrogen radicals than usual, which would be activated each spring by sunlight.
But while most scientists agree that atmospheric chemistry and dynamics are major causes, the increased scrutiny of the Antarctic atmosphere following the discovery of the hole has seriously undercut the sunspot theory. Data from Punta Arenas, says Robert Watson, a NASA scientist involved in that study, made the verdict all but final. Nitrogen and ozone levels were down, but concentrations of chlorine monoxide were 100 times as great as equivalent levels at temperate latitudes. Says Watson: "We can forget the solar theories. We can no longer debate that chlorine monoxide exists and that its abundance is high enough to destroy ozone, if our understanding of the catalytic cycle is correct. We need to go back to the lab and resolve the uncertainty."
That is not all. Scientists are still not completely sure why the hole remains centered on the Antarctic or why the depletion is so severe. It may have to do with the peculiar nature of Antarctic weather. In winter the stratosphere over the region is actually sealed off from the rest of the world by the strong winds that swirl around it, forming an all but impenetrable vortex. Says Cicerone: "Looking down at the South Pole is like watching fluid draining in a sink. It's like an isolated reactor tank. All kinds of mischief can occur."
One likely source of mischief making: clouds of ice particles in the polar stratosphere. Explains Rowland: "Mostly, you don't get clouds in the stratosphere because most of the water has been frozen out earlier. But if the temperature gets low enough, you start freezing out the rest." Indeed, ice may prove to be a central cause of the ozone hole, since it provides surfaces for a kind of chemistry only recently associated with reactions in the atmosphere. In a gaseous state, molecules bounce around and eventually some hit one another. But adding a surface for the molecules to collect on speeds up the reactions considerably.